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Views: 487 | Replies: 9 | Solved
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fread() doesn't work when 're-used' in a for( ; ;) function

  #1  
Jun 16th, 2008
Hey, i'm trying to sort direct in a file.
The problem is that when i read the first time an int from it,it works! doh.
but when it reads for the second time, it doesn't. ( Neither the swap works.
No signs of errors from the compiler.


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language Syntax (Toggle Plain Text) 
  1. FILE *fp=fopen("element.txt","rw+");
  2. fseek(fp, SEEK_SET, 0);
  3.  
  4.  
  5. for(i=0;i<n;i++){
  6.   fgetpos(fp, &filepos);
  7.   fread(&t, sizeof(int), 1, fp);
  8.  for(j=i+1;j<n-1;j++){
  9.    fgetpos(fp, &filepos2);//t2=NULL;
  10.    fread(&t2, sizeof(int), 1, fp); //<< 'broken', t2's value does not change
  11.    if(t>t2){
  12. 	 fsetpos(fp, &filepos);          //
  13. 	 fwrite(&t2,sizeof(t2),1,fp);  // <<All of this doesn't work also.
  14. 	 fsetpos(fp, &filepos2);        //File remains the same.
  15. 	 fwrite(&t,sizeof(t),1,fp);}   //
  16.  }
  17. }
  18. fclose(fp);
Last edited by Alex_ : Jun 16th, 2008 at 5:11 pm.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #2  
Jun 16th, 2008
You could try checking return values such that the code itself tells you what the problem is.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #3  
Jun 16th, 2008
How's t and t2 declared?

FILE *fp=fopen("element.txt","rw+"); How about checking that the file was opened?
Help with Code Tags 
if ( fp == NULL )
{
     <error...>
}

Upon success fread() returns the amount of elements read in a form of size_t object; why don't you check that?

Help with Code Tags 
size_t result = 0;
result = fread(&t, sizeof(int), 1, fp);
if( result > 0 )
{
     <do something...>
}
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #4  
Jun 16th, 2008
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Originally Posted by Dave Sinkula View Post
You could try checking return values such that the code itself tells you what the problem is.

Done did that.
The function returns 0;which means no good. On succes it gives 1.
I have modified the code.Now on the third time the function does not work.(Works only twice).

Help with Code Tags
language Syntax (Toggle Plain Text) 
  1. int i,j,t,t2;

  2. fpos_t filepos,filepos2;

  3. FILE *fp=fopen("element.txt","rw+");

  4. fseek(fp, SEEK_SET, 0);

  5.  
  6. do{filepos+=2*i;

  7.   pos=ftell(fp);

  8.   fread(&t, sizeof(int), 1, fp);

  9.  do{j=i+1;

  10. 	filepos2+=2*j;

  11.    fread(&t2, sizeof(int), 1, fp);//<<Broken when the compiler hits it for the 3rd time.

  12.    pos=ftell(fp);

  13.    if(t>t2){

  14. 	 fsetpos(fp, &filepos);

  15. 	 fwrite(&t2,sizeof(t2),1,fp); 

  16. 	 fsetpos(fp, &filepos2);

  17. 	 fwrite(&t,sizeof(t),1,fp);t=t2;}

  18.  j++;fclose(fp);FILE *fp=fopen("element.txt","rw+");}while(j<n-1);

  19. i++;}while(i<n);
Last edited by Alex_ : Jun 16th, 2008 at 6:25 pm.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #5  
Jun 16th, 2008
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Originally Posted by Aia View Post
How's t and t2 declared?

FILE *fp=fopen("element.txt","rw+"); How about checking that the file was opened?
if ( fp == NULL )
{
<error...>
}


If fread worked for 1st and 2nd time, then surely the file was opened.

t and t2 are integers.
Maybe the problem is in wich mode i open the file? like "wr+"
Last edited by Alex_ : Jun 16th, 2008 at 6:21 pm.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #6  
Jun 16th, 2008
http://www.c-faq.com/stdio/fupdate.html

[edit]Hm. From n1124:
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When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters endof- file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.
Last edited by Dave Sinkula : Jun 16th, 2008 at 6:32 pm.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #7  
Jun 16th, 2008
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Originally Posted by Alex_ View Post
If fread worked for 1st and 2nd time, then surely the file was opened.

Regardless, I was giving you a suggestion towards good code practice.

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Originally Posted by Alex_ View Post
Maybe the problem is in wich mode i open the file? like "wr+"

fread() and fwrite() requires you to open the file in binary mode, appending a `b' to it.
i.e. "r" would be "rb"
Last edited by Aia : Jun 16th, 2008 at 6:42 pm.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #8  
Jun 16th, 2008
Evrica! It works!
Thank you Deave Sinkula and Aia! Without you i wouldn't have understood the mistake...by now.
Tomorrow i'll get a 9 (nine) for this program!

One last question: Why does the sorting goes well till n-1,but at n it just copies n-1?
I.e: 5 4 6 3 7 2 8 1 9
sorted: 1 2 3 4 5 6 7 8 8

Help with Code Tags 
FILE *fp=fopen("element.txt","rb+");
fseek(fp, SEEK_SET, 0);

while(i<n-1){filepos=2*i;
  fsetpos(fp, &filepos);
  fread(&t, sizeof(int), 1, fp);

  j=i+1;
 while(j<n){
	filepos2=2*j;
	fsetpos(fp, &filepos2);
	fread(&t2, sizeof(int), 1, fp);
  if(t>t2){
	 fsetpos(fp, &filepos);
	 fwrite(&t2,sizeof(t2),1,fp);
	 fsetpos(fp, &filepos2);
	 fwrite(&t,sizeof(t),1,fp);t=t2;}
 ++j;}
++i;}
Last edited by Alex_ : Jun 16th, 2008 at 7:34 pm.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #9  
Jun 16th, 2008
You would have to post your modified code, to answer that last question.
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Re: fread() doesn't work when 're-used' in a for( ; ;) function

  #10  
Jun 16th, 2008
Topic Solved by Aia
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